when gisting stdin default to filename 'a.rb' instead of '-'

[phallstrom]

fixes #235

test plan:

• ensure you're signed in (ie. have a token)

% jot 5 | gist

check gist, filename should be 'a.rb'

% gist Gemfile

check gist, filename should be 'Gemfile'

[ConradIrwin]

@phallstrom I think it should always default to - or if file extensions are important, then maybea.txt (I don't want to assume everyone's using ruby just because the tool is written in ruby!).

This change breaks the ability to create a gist combining both an existing file and stdin:

gist header.txt -

[phallstrom]

@ConradIrwin Are you cool changing the previous default of a.rb to - as well? If you didn't want - looks like the website uses gistfile.txt. Let me know...

[ConradIrwin]

Yup, I'm ok with -. gistfile.txt would be ok too. I'm not too fussed.

[phallstrom]

@ConradIrwin code modified to use gistfile1.txt and support both a file and stdin. Thanks!

[ConradIrwin]

Thanks again!